1. Field of the Invention
The present invention generally relates to an apparatus for synthesizing diamond via a chemical vapor deposition process using hot metal filament and a method thereof. In particularly, the present invention relates to an apparatus and a method for using the hot filament repeatedly without breakage, in comparison with the conventional apparatus that the filament is limited in one use.
2. Description of the Prior Art
A conventional apparatus for forming diamond via a chemical vapor deposition process using hot metal filament puts its focus on preventing hot metal filament from being (slacked while the filament is limited in one use.
However, the present invention is related to an apparatus completely preventing the filament from being slacked at high temperature, and it provides an apparatus for using the hot filament repeatedly without breakage and a method thereof.
Diamond has the high industrial applicability because of its excellent hardness, thermal conductivity, electric insulating property, optical property and other chemically stable physical characteristics.
Particularly, the importance of diamond has been emphasized in the 1980s since a chemical vapor deposition process based on hydrogen and methane gas is used.
As a result, various methods for efficiently sythesizing diamond via a chemical vapor deposition process are suggested. Notable among these methods is a method for depositing diamond by chemical vapor deposition using hot metal filament. It is simple in its equipment and economical because diamond can be synthesized in larger sizes. Due to these characteristics, this method is actively studied.
Generally, it has been known that diamond can be synthesized when metal filament is maintained at a temperature of 1800˜2500° C. but a base substrate at a temperature of 700˜1100° C., and a gas is then injected during the process of synthesizing diamond using hot filament.
In other words, the above-described method has a simpler structure than the conventional one.
However, the method has special problems because metal filament needs being maintained at a high temperature. To overcome those problems is the most important and crucial technique for synthesizing diamond using hot metal filament. Accordingly, those problems will be examined hereinafter.
Diamond for being synthesized needs being maintained at a temperature of over 2000° C. Here, the length of metal filament is continuously changed by thermal expansion.
Particularly, the filament is slacked at a high temperature, because the filament is deformed by its weight at a high temperature.
In other words, it is necessary to control properly the temperature of diamond for being synthesized. Additionally, it is also important to maintain the region for synthesizing diamond at a settled temperature for hours because the diamond has the slow growth rate of 0.5˜2 μm/h.
However, the phenomenon wherein the filament is slacked or broken makes it impossible to control the temperature of diamond for being synthesized. The phenomenon also causes the temperature gradient, and makes it impossible to regulate the distance between the filament and the base substrate wherein diamond is synthesized. As a result, diamond cannot be synthesized.
Another problem is the carburization of metal filament while the process of synthesizing diamond is performed under the carbonizing (i.e. methane gas) condition.
In other words, if the metal filament is previously carbonized, its volume is expanded and its form is seriously distorted. This phenomenon causes the filament to be changed in its length and to have the internal stress. First of all, the previously carbonized filament has the same brittleness such as that of ceramics. As a result, the filament tends to be broken by external force or impact.
Particularly, if it is considered that the general diameter of the filament used in synthesizing diamond is 0.02˜0.5 mm, it is difficult to maintain the carbonized filament without breakage.
In other words, tungsten is generally used as the filament. Here, tungsten carbide is formed by the carburization of the filament, thereby resulting in the brittleness of ceramics.
Due to this problem, the brittle filament is broken by its expansion and contraction when the filament used in synthesizing diamond is heated to a high temperature and then cooled to a ambient temperature.
To control and prevent the problem is very important for a chemical vapor deposition process.
Additionally, the brittleness causes the filament to be limited in one use for synthesizing diamond.
The above-described problems of the filament cause more serious problems in designing the equipment for large area.
There have been various methods to prevent the hot filament from breakage by the brittleness resulting from the slack and carburization of the hot filament. Among these methods, the U.S. patent application Ser. No. 4,953,499 provides a pre-stressed curved filament to endure the thermal expansion and the carburization of the filament.
However, in this method, the filament is easily broken when it is pre-stressed. The previously curved filament has the internal stress, therefor it tends to be arranged to an undesired direction and form by the rise of temperature.
Particularly, if the curved filament is placed on a mis-fixed electrode, it is changed into an undesired size and form by the carburization of the filament. As a result, it is impossible to regulate the changed filament.
The U.S. Pat. No. 4,970,986 provides another method that copes with thermal expansion of the metal filament by mounting individual spring on the filament to previously have a tensile stress.
However, in this method, since the spring is once. mounted inside of the apparatus for synthesizing diamond, it is impossible to regulate the spring while synthesizing diamond. Additionally, since the spring has the excessive stress, the extremely thin filament is easily broken.
Although a plug is mounted on the opposite side to regulate the stress, it is still difficult to regulate properly the individual stress of a plurality of filaments.
The U.S. Pat. No. 4,958,592 provides a method to regulate the transformation resulting from the thermal expansion and carburization of the filament by installing the complicated counterbalancing weight assembly on the filament which is located perpendicularly and generates heat.
However, this method still has many problems. The counterbalancing weight assembly has a bar which applies a tension to the filament. The bar is slipped down by a bearing and then connected to the end of the counterbalancing bar (lever). Counterbalancing weights are placed on the opposite side of this counterbalancing bar.
A supporting rod of the lever is hung on the synthetic equipment by a string. This counterbalancing weight assembly is easily fluctuated in every direction even by near small impact. As a result, it is impossible to adjust the filament.
Because a tensile bar is moved by the bearing, it is possible to control the weight in an up-and-down or circumferential movement but impossible to adjust the three-dimensional movement (swing or vibration) of the weight.
First of all, a perpendicularly mounted filament is connected to the complicated counterbalancing weight assembly (tensile bars, levers, weights) hung to the synthetic equipment. Accordingly, it is improper structure in synthesizing diamond with large size using a plurality of filaments.
The U.S. Pat. No. 5,833,753 and U.S. Pat. No. 5,997,650 provide a method to move electrodes (movable electrodes) wherein filaments are arranged and to regulate a tensile stress by installing a force regulator on the movable electrodes.
This method needs the complicated subsidiary equipment and the precision control to fix a plurality of filaments having the identical length respectively on the movable electrodes.
The Korean Patent No. 100,286 provides a method of absorbing the change in length of the filament during its thermal expansion by forming a hole whereon the end of the filament is slipped to electrodes. In this method, a supporting bar is mounted between electrodes lest the filament should be slacked. However, this method has shortcoming that a lot of supporting bars is required, even if the length of filaments gradually increases.
The Korean Patent Application No. 105,943 provides a method of preventing the filament from being slackened at a high temperature. In this method, the filament is formed into double lines twisted with several single ones. These filaments having a spiral form are contracted parallel with the lines during its carburization. In this way, it is possible to prevent the filament from being slackened. However, it is desirable to use short filaments. If the filaments become longer, it is impossible to prevent the filament from being slackened by its weight because of the limit in its contraction.
The above-described conventional methods need all intricate subsidiary equipment. Particularly, these methods place their emphasis on the prevention of slackening filaments and they are not considered in usage count of filaments.
In other words, those methods relate to the control and installment of filaments for synthesizing diamond limited in one use. Here, the possibility of using the filaments repeatedly is excluded. The filaments have a problem in the structure wherein they cannot be used repeatedly.
Although the Korean Patent No. 100,286 and 105943 among those methods suggest the possibility of using the filaments repeatedly, but still have a structural problem in synthesizing diamond with large size.